The cleaning of artworks often involves the removal of dirt from damaged surfaces or fragile substrates. The traditional cleaning techniques such as water, chemical and mechanical cleaning can result in permanent damage of the substrates.
In many geographical areas where granite is widely available, traditional architecture in historical buildings utilizes granite stone composed by feldspar, quartz, and mica. As valuable monuments of significant cultural heritage started to present a disintegration of the granite stone in the outdoor walls, owing to chemical erosion, some restorers decided to apply molten beeswax on their granite surfaces to stop the imminent deterioration of the monuments. At first, this treatment was a good solution to consolidate the stone, but with time it resulted counterproductive. The wax layer on the surface does prevent rain water from penetrating the stone surface, but granite catches the damp from all of its uncoated surfaces. Through the irregularities in the beeswax film such as pores, water evaporation from the surface takes place, leading to a gradient of salt concentration inside the stone, and the movement of the salt solutions from the interior of the rocky mass towards the surface, hence precipitation of these salts occurs where the solubility product has been exceeded. Owing to crystallization of salts, and their accumulation beneath the beeswax layer, an intense surface disintegration of granite can be observed, so that the conservation of sculptured details of emblematic monuments is at risk.
Due to the delicate state of these stone monuments, a non-contact, selective, and environmental friendly cleaning technique is necessary to remove the waxy material. It is of critical importance to avoid the beeswax melting and its advance deeper into the granite cracks. Granite stone studies confirm that the porous stone network contains micro (r<3 μm) and macrofissures (r≧3 μm), however, the most abundant microfissures in Roan granite are those having widths of tenths of micron. Since the beeswax was applied by melting on the granite surface, it has penetrated the intricate granitic porous system, resulting in a rather complex “wax-stone” system. Consequently, the development of nondestructive methods for removing the wax with minimal impact to the historical piece presents several difficult problems to overcome and traditional chemical and mechanical cleaning techniques are not adequate. For instance, even the characterization of the beeswax thickness at the stone surface is very challenging. Several methods have been evaluated as means for beeswax characterization on artwork, such as gas chromatography, differential scanning calorimetry, Fourier transform infrared Spectroscopy, and X-ray diffraction. However, the main drawback of these methods is their destructive nature, since they require significant amounts of sample of the surface to be extracted. Therefore, they cannot be used for assessing the thickness of a wax layer on the surface of granite, as needed in the restoration field, especially if this novel method should eventually be used in-situ to evaluate in real time the efficiency of new cleaning techniques.